Hydrophilized porous polyolefin membrane and production process thereof

ABSTRACT

The present invention discloses a hydrophilized porous polyolefin membrane with a polymer of a monomer, which has an HLB value of 2-20, held on at least a part of the pore walls of a porous polyolefin membrane as well as its production process. This hydrophilized porous polyolefin membrane has long-lasting hydrophilicity and good mechanical strength.

This is a division of application Ser. No. 928,163 filed Nov. 7, 1986,U.S. Pat. No. 4,678,813, July 7, 1987.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a porous membrane permitting permeation of ahydrophilic liquid therethrough and a production process thereof.

(2) Description of the Prior Art

The fields of application of porous polyolefin membranes is growingrapidly due to their excellent mechanical properties and chemicalresistance. Porous polyolefin membranes are however hydrophobic, andwhen used as is water permeates therethrough with difficulty. Ahydrophilizing treatment is therefore indispensable to have hydrophilicliquids including water to permeate therethrough. A variety of methodshave been studied with a view toward imparting hydrophilicity throughsurface modification of polyolefin membranes. Hydrophilizing methods,which have been proposed for film-like materials featuring smoothsurfaces, cannot be simply applied to impart hydrophilicity to porousmembranes having complex surface configurations.

As hydrophilizing methods for porous polyolefin membranes, there havebeen known the organic solvent wetting and water substituting method,the physical adsorption method, the chemical surface modification methodand other methods. In the organic solvent wetting and water substitutingmethod, the entire surface of a porous polyolefin membrane, inclusive ofminute pores, is subjected to a wetting treatment with an organicsolvent having good miscibility with water such as an alcohol or ketone,followed by the substitution of water for the organic solvent. If wateris once lost from minute pores during storage or use, the partcontaining these water-free minute pores regains hydrophobicity and nolonger permits the permeation of water therethrough. Accordingly, it isalways necessary to keep water around the porous membrane in the abovemethod. The porous membrane is therefore cumbersome to handle. Accordingto the physical adsorption method, a hydrophilic material such aspolyethylene glycol or a surfactant is adsorbed on the pore walls of aporous membrane so as to impart hydrophilicity to the porous membrane(see, for example, Japanese Patent Laid-Open Nos. 153872/1979 and24732/1984). Although this method is easy to practice, the hydrophilicmaterial drops off if the resulting porous membrane is used over a longperiod of time. Therefore, this method cannot be regarded as asatisfactory hydrophilizing method.

As a chemical surface modification method, it has been proposed, forexample, to expose a porous membrane to radiation while holding ahydrophilic monomer on the surface of the membrane (Japanese PatentLaid-Open No. 38333/1981) or to subject the porous structure of ahydrophobic resin to a plasma treatment in a state impregnated with awater-soluble high-molecular material and a surfactant (Japanese PatentLaid-Open No. 157437/1981). These methods are however accompanied by oneor more problems. It is difficult to impart uniform hydrophilicity inthe direction of the thickness of a membrane, no matter which of thesemethods is relied upon. If one attempts to apply a hydrophilizingtreatment uniformly over the entire thickness of a porous membrane whenthe membrane has a large thickness or is in the form of hollow fiber,the mechanical strength of the matrix of the porous membrane isunavoidably reduced.

As has been described above, no effective hydrophilizing method has beenestablished to date for porous polyolefin membranes.

SUMMARY OF THE INVENTION

An object of this invention is to provide a porous polyolefin membraneimparted with hydrophilicity of excellent durability over almost all ofthe pore walls of the membrane and having sufficient mechanicalstrength.

In one aspect of this invention, there is thus provided a hydrophilizedporous polyolefine membrane, wherein a polymer of a monomer having anHLB value of 2-20 is held on at least a part of the pore walls of astarting porous membrane of a polyolefin.

In another aspect of this invention, there is also provided a processfor the production of the aforementioned hydrophilized porous polyolefinmembrane, which comprises the steps of holding a monomer which has anHLB value of 2-20 and contains at least one unsaturated polymerizablebond and a polymerization catalyst on at least a part of the pore wallsof a starting porous polyolefin membrane and heating them to polymerizethe monomer

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, the polyolefin forming the porous membraneincludes a polymer or copolymer composed principally (80 wt. % or more)of one or more monomers selected from the group consisting of ethylene,propylene, 4-methyl-1-pentene and 3-methyl-1-butene and a fluorinatedpolyolefin.

The starting porous membrane may be in any form such as a hollow fibermembrane, planar membrane or tubular membrane. Although starting porousmembranes having various pore sizes may be employed depend on the enduse, the preferred starting porous membranes may include those having amembrane thickness of about 20-200 μm, a porosity of about 20-90 vol. %,a water permeability of about 0.001-10 l/min.hr.mmHg as measured by thealcohol-dependent hydrophilizing method, and a pore size of about 0.01-5μm.

The monomer (hereinafter called "surface active monomer") useful in thepractice of this invention, the HLB value of which ranges from 2 to 20,contains at least one unsaturated polymerizable bond, at least onehydrophilic moiety and at least one hydrophobic moiety in its molecule.It is immaterial whether the monomer is ionic or nonionic. Although noparticular limitation is imposed on its molecular weight, it ispreferably 10,000 or less, more preferably 5,000 or less, and mostpreferably 3,000 or less. As the unsaturated polymerizable bond whichthe monomer contains, may be-mentioned a double bond such as vinyl bondor allyl bond or a triple bond such as diacetylene bond. Illustrativeexamples of the hydrophilic moiety include ethylene oxide, phosphoricesters, sulfonic group and its salts, hydroxyl group, carboxylic acidgroups and their salts and quaternary ammonium group. On the other hand,exemplary hydrophobic moieties include hydrocarbon chains such asmethylene group, alkyl groups, phenyl group, vinyl group, allyl groupand acetylene bond and C₃ -- and higher alkylene oxides such aspropylene oxide and butene oxide.

As specific examples of the monomer, compounds represented by any one ofthe following general formulae (1)-(13) may be mentioned by way ofexample.

    H.sub.2 C═CR.sup.1 COO(EO).sub.l (R.sup.2).sub.m (EO).sub.n COCR.sup.1 ═CH.sub.2                                             ( 1)

    H.sub.2 C═CR.sup.1 COO(R.sup.2).sub.l (EO).sub.m (R.sup.2).sub.n COCR.sup.1 ═CH.sub.2                                  ( 2)

    H.sub.2 C═CR.sup.1 COO(EO).sub.l (R.sup.2).sub.m (EO).sub.n R.sup.3 ( 3)

    H.sub.2 C═CR.sup.1 COO(R.sup.2).sub.l (EO).sub.m (R.sup.2).sub.n R.sup.3                                                   ( 4)

    H.sub.2 C═CR.sup.1 COO(EO).sub.l (R.sup.2).sub.m COCR.sup.1 ═CH.sub.2                                             ( 5)

    H.sub.2 C═CR.sup.1 COO((CH.sub.2).sub.4 O).sub.k COCR.sup.1 ═CH.sub.2                                             ( 6) ##STR1##

    H.sub.2 C═CR.sup.1 COO(EO).sub.l PO(OH)OR.sup.4        ( 8)

    H.sub.2 C═CR.sup.1 CH.sub.2 (OCH.sub.2 CH(OH)CH.sub.2).sub.h SO.sub.3 M (9) ##STR2##

    H.sub.2 C═CR.sup.1 COO(EO).sub.l COCR.sup.1 ═CH.sub.2 ( 11)

    CH.sub.3 (CH.sub.2).sub.i C.tbd.CC.tbd.CCOOH               (12)

    CH.sub.3 (CH.sub.2).sub.i C.tbd.CC.tbd.C(CH.sub.2).sub.j COOM (13)

In the general formulae (1)-(13), l, m, n, k, h, i, j, EO, R¹, R², R³,R⁴, R⁵, R⁶ and M are defined as given below. Depending on the manner ofselection of the chain length of ethylene oxide or propylene oxide andthe chain length of each alkyl group, some compounds outside the scopeof the present invention may also be included. These chain lengths arehowever suitably chosen in the present invention, and compounds, the HLBvalues of which fall within the range of 2-20, are employed in thepresent invention.

l, m, n, k, h: integer of 1-100

i, j: integer of 1-20

EO: --CH₂ CH₂ O--

R¹ : H or CH₃ -- ##STR3## R³, R⁴, R⁵, R⁶ : alkyl group having 4-20carbon atoms M: alkali metal

Although the HLB value of the surface active monomer useful in thepractice of this invention ranges from 2 to 20, a range of 5-15 isparticularly preferred. It should be noted that, as HLB values referredto in the present invention, those determined by the method proposed byDavies [Progress The Second International Congress of Surface Activity,1, 426, (1957)] were adopted.

In the hydrophilized porous membrane of this invention, a hydrophilicpolymer composed of a monomer having an HLB value of 2-20 is held on atleast a part of the pore walls of a starting porous polyolefin membrane.It is sufficient if the hydrophilic polymer is held on the pore walls tosuch an extent that when the porous membrane is used by allowing waterto permeate through its pores under the usual intermembrane pressuredifference, an acceptable flow rate is achieved through the membrane. Itis not absolutely necessary to cover the entire pore walls with thehydrophilic polymer. Furthermore, the hydrophilic polymer may also beheld on the outer surfaces of the porous membrane. The term "held" asused herein means that the hydrophilic polymer is bonded or otherwiseadhered firmly to the pore walls to such a degree that the hydrophilicpolymer easily drops off in the course of storage or use of the porousmembrane. The hydrophilic polymer may be graft-polymerized to the porewalls or may firmly adhere to the fine porous area and stick there owingto its anchorage in the individual pores. As a further alternative, thehydrophilic polymer may also be held by both graft-polymerization andanchorage.

In the present invention, the hydrophilic polymer composed of themonomer, the HLB value of which ranges from 2 to 20, is characterized inthat it is practically insoluble in water and its degree ofhydrophilicity is great compared to polyolefins. A porous membrane withthe hydrophilic polymer held on at least a part of pore the walls of astarting porous polyolefin membrane has substantially permanenthydrophilicity.

On the other hand, monomers, the HLB values of which are greater than20, have little interaction with polyolefins. It is hence difficult tohold their polymers on the pore walls of starting porous membranes andmoreover, the polymers of these monomers have large solubility in water.If porous membranes which have been obtained.by holding these polymerson the pore walls of starting porous polyolefin membranes are used inwater, the polymers dissolve in water and hence are lost. There is thusthe danger that the properties of the pore walls of the porousmembranes, on which pore walls the polymers are held at the beginning,will change from hydrophilic properties to hydrophobic properties.Further, polymers formed of monomers having HLB values smaller than 2are insoluble in water. The degrees of their hydrophobicity is howeverso great that the pore walls of porous polyolefin membranes, on whichpore walls the polymers are held, do not show any hydrophilicity.

It may be considered to convert the polymers of monomers, the HLB valuesof which exceed 20, into polymers with crosslinked structures introducedtherein with a view toward lowering the water-solubility of the formerpolymers. This however leads to the drawbacks that the thus-crosslinkedpolymers will swell in water and that the pore sizes of porous membraneswill become smaller.

The amount of the hydrophilic polymer held on at least a part of thepore walls of a porous polyolefin membrane according to this inventionmay be 0.5-100 wt. % preferably 0.5-50 wt. %, more preferably 1-30 wt.%, and most preferably 2-15 wt. %.

A variety of processes may be employed to obtain the porous membranes ofthis invention. However, they are usually obtained by polymerizing asurface-active monomer while holding the surface-active monomer in astate adhered to the pore walls of a porous polyolefin membrane.

As a method for causing the surface-active monomer to adhere to theporous membrane, the following method may be employed by way of example.A solution of a surface-active monomer and if necessary, apolymerization catalyst dissolved in a suitable solvent such as anorganic solvent or water, is prepared. After impregnating a startingporous polyolefin membrane with the above solution by immersing theporous polyolefin membrane in the solution or by fabricating a membranemodule with the porous polyolefin membrane and then causing the solutionto penetrate under pressure into the porous polyolefin membrane, thesolvent is cause to evaporate off. It is also possible to cause thesurface-active monomer to adhere almost uniformly over the entiresurface of the porous membrane without plugging the pores of the porousmembrane by using the surface-active monomer in a form diluted with asolvent. The amount of the surface-active monomer to be adhered can beadjusted by changing the concentration of the surface-active monomer inthe solution.

The surface-active monomer held on the pore walls of the porouspolyolefin membrane in the above-described manner can then bepolymerized by, for example, heat polymerization, photopolymerization orradiation polymerization, whereby a hydrophilic polymer can be formed onthe pore walls of the porous membrane.

The solvent useful upon preparation of the above-described solutionincludes water or an organic solvent which has a boiling point lowerthan the surface-active monomer and can dissolve the surface-activemonomer therein. When a polymerization catalyst is added, it isdesirable to use a solvent which can also dissolve the polymerizationcatalyst. When water is employed as the solvent, it is possible todisperse even a polymerization catalyst which is inherently insoluble inwater as minute particles in water owing to the surface activity of thesurface-active monomer. The polymerization catalyst can thus actpractically as if dissolved. It is therefore possible to use water asthe solvent even when a water-insoluble polymerization catalyst isadded.

Since the surface of a porous polyolefin membrane is hydrophobic, asurface-active monomer tends to be adsorbed on the pore walls with itshydrophilic groups oriented outward when an aqueous solution containingthe surface-active monomer penetrates into the pores. If thesurface-active monomer is fixed in this state by polymerization,hydrophilicity can be imparted with extremely high efficiency. Use ofwater as a solvent is preferable from the viewpoint of workability,maintenance of good working environment and other conditions.

By contrast, use of an organic solvent as the solvent has the merit thatthe resulting solution is allowed to penetrate into pores of a porouspolyolefin membrane in a short period of time and that the solvent canbe removed with ease from the pores.

Even when the surface-active monomer is polymerized in a state orientedat random on the pore walls instead of making use of the above-mentionedoriented adsorption, the resulting hydrophilic polymer has a greatdegree of hydrophilicity compared to polyolefins. Compared with porewalls not holding the hydrophilic polymer thereon, pore walls with thehydrophilic polymer held thereon have higher hydrophilicity.

As the polymerization catalyst employed upon polymerization of thesurface-active monomer, it is possible to use any of the variousperoxides such as azo compounds and redox initiators which are known asradical polymerization initiators for heat polymerization. Exemplarypolymerization catalysts include benzoyl peroxide, dicumyl peroxide,azobisisobutyronitrile, azomethane and combinations of metal salts andhydrogen peroxide.

In the case of photopolymerization, it is possible to usephotopolymerization catalysts, for example, carbonyl compounds such asdiacetyl, benzil, benzaldehyde and cyclohexanone; ketone compounds suchas benzophenone, parachlorobenzophenone, 2,4-dichlorobenzophenone,cyanobenzophenone, benzophenone sulfide and acetophenone; anthraquinonecompounds such as anthraquinone and 2-ethylanthraquinone; azo compoundssuch as azobisisobutyronitrile and azomethane; benzoyl peroxide; uranylbiacetylnitrate; benzyl dimethyl ketal; dibenzothiazolyl sulfide; eosin;erythrosine; neutral red; and victoria blue.

Organic solvents capable of dissolving both the surface-active monomerand the polymerization catalyst include methanol, ethanol, butanol,propanol, chloroform, acetic acid, toluene, benzene, acetone, methylethyl ketone, methyl isobutyl ketone, tetrahydrofuran, dimethylformamide, dimethyl acetamide and dimethyl sulfoxide. Although noparticular limitation is imposed on the boiling point of these organicsolvents, the boiling point is preferably about 100° C. or lower andmore preferably about 80° C. or lower because this lower boiling pointfacilitates evaporation of the solvent upon its removal prior to thepolymerization step. Regarding the proportions of the surface-activemonomer, solvent and polymerization catalyst in a solution, it ispossible to employ conditions such that the solvent and polymerizationcatalyst are contained in amounts of 50-10,000 parts by weight and0.001-100 parts by weight, respectively, per 100 parts by weight of thesurface-active monomer. As more preferable conditions, the solvent andpolymerization catalyst will be in amounts of 500-10,000 parts by weightand 0.01-30 parts by weight respectively, both, per 100 parts by weightof the surface-active monomer.

When a starting porous polyolefin membrane is subjected to an immersionor penetration treatment by using the above-described solution, theimmersion or penetration time may be about 0.5 second-30 minutes. Thistreatment can be effected in a shorter period of time as the wettingcharacteristics of the solution for the porous polyolefin membranebecome better. After the dipping treatment, the accompanying extrasolution is removed and if necessary the solvent penetrated in the poresis caused to evaporate, followed by a polymerization step. If thetemperature is too high upon evaporation of the solvent, thepolymerization catalyst may be decomposed and the polymerization mayhence be caused to proceed partially while the solvent still remains. Asa result, the polymerization may take place not on the pore walls of theporous membrane but within its pores and some pores may hence beplugged. It is therefore not desirable to use too high temperature. Forthis reason, about 10°-40° C. is preferred as the temperature uponremoval of the solvent.

In heat polymerization, the polymerization temperature is above thedecomposition temperature of the above-mentioned polymerizationcatalyst. It is also desirable to choose a temperature in which themembrane structure of the porous polyolefin membrane is not changed andthe matrix of the membrane is not damaged. It is generally preferable touse a temperature of about 30°-100° C. Although the heating time dependson the type of polymerization catalyst and the heating temperature, itis generally about 1 minute-5 hours and more preferably about 5minutes-60 minutes in a batch process. Since the heat transferefficiency is higher in a continuous process, the polymerization can beachieved in a shorter period of time. Therefore, the heating time mayusually be about 10 seconds-60 minutes with about 20 seconds-10 minutesbeing preferred.

In photopolymerization, ultraviolet rays or visible light can be used asthe light to be irradiated. Ultraviolet rays having large energy areparticularly preferred. As the ultraviolet ray source, a low-pressuremercury lamp, high-pressure mercury lamp, xenon lamp, arc lamp or thelike may be used.

Light irradiation conditions are dependent on the intensity of light tobe irradiated. It is difficult to impart sufficient hydrophilicity at alow irradiation intensity. On the other hand, porous polyolefinmembranes are significantly damaged at high irradiation intensity. It ishence essential to choose suitable light irradiation conditions. When amercury vapor lamp is used as an exemplary light source, it is necessaryto set the input at about 10-300 W/cm and to irradiate light for about0.5-300 seconds at a distance of about 10-50 cm so that a porouspolyolefin membrane is exposed to light with energy of about 0.001-10joule/cm² or more preferably about 0.05-1 joule/cm².

Radiation polymerization can be conducted, for example, by irradiatingelectron beams to about 10-50 Mrad at a temperature below 120° C., morepreferably below 100° C., by means of an electron beam irradiationapparatus.

The surface-active monomer held on the pore walls of the starting porousmembrane is graft-polymerized to the porous membrane or is polymerizedon the surfaces of the porous membrane by any one of thesepolymerization techniques. At least a part of the pore walls of theporous membrane is therefore covered by the resulting polymer.

Among the above-mentioned polymerization techniques, it is mostpreferable to effect the polymerization by thermal energy. Since use ofthe thermal energy allows even pore portions of the porous membraneuniformly to be heated to a desired temperature, the surface-activemonomer can be uniformly polymerized over the entire pore walls on whichthe surface-active monomer is held. Heat polymerization has anotheradvantage in that the polymerization can be achieved withoutmodification of the membrane structure and deterioration of the membranematrix if the polymerization temperature is suitably chosen. Bycontrast, the use of light energy involves the problem that the lightcannot sufficiently reach the pore portions of the porous membrane dueto scattering of the light. Furthermore, the use of radiation energy isaccompanied by the drawback that the membrane matrix is liable toaccelerated deterioration.

If oxygen exists in the atmosphere upon polymerization, thepolymerization reaction is significantly impaired. It is thereforedesirable to effect the polymerization in a substantially oxygen-freestate, for example in an inert gas atmosphere like a nitrogen gasatmosphere or in vacuo.

It is also desirable to remove unnecessary materials such as unreactedmonomer or free polymer with an appropriate solvent subsequent to theformation of the polymer. Where the hydrophilic polymer formed on thepore walls of the porous membrane is an uncross-linked polymer, it ispreferable to use a solvent that dissolves the unreacted monomer butdoes not dissolve the hydrophilic polymer. Where the hydrophilic polymeris a crosslinked polymer, a solvent which can dissolve the unreactedmonomer is used.

The porous membrane of this invention can be obtained in theabove-described manner.

The individual steps of the process of this invention have beenseparately described above. It should however be noted that suchindividual steps as holding of the surface-active monomer on the porewalls of a porous membrane, removal of the solvent, polymerization andwashing after the polymerization can be performed continuously in thepresent invention.

The present invention will hereinafter be described specifically by thefollowing Examples. In each Examples, a porous membrane with slit-likespacings, which were defined by fibriles and knots extendedthree-dimensionally, was used and the pore size of the porous membranewas defined in terms of the average width and length of the slit-likespacings. Water penetration pressure, water permeability, and waterpermeability by the alcohol-dependent hydrophilizing method were eachmeasured in accordance with the following methods by fabricating testmembrane modules each of which had an effective membrane area of 163cm². In addition, the amount of polymer held and the surface coverage(%) were also measured by the following methods:

(1) Water penetration pressure:

Water of 25° C. was fed from one side (the inside of hollow fibers inthe case of a hollow fiber membrane) of a test membrane module whileraising the water pressure at a rate of 0.1 kg/cm² per minute. Waterpressures were measured when the cumulative quantity of penetrated waterhad reached 30 ml and 50 ml separately. The water pressures andquantities of penetrated water were plotted along the axis of theabscissa and the axis of the ordinate respectively. The pressure at thecrossing point between the straight line, which connected thethus-plotted two points, and the axis of abscissas was determined. Thepressure was employed as the water penetration pressure.

(2) Water permeability:

Water of 25° C. was caused to flow from one side of a test membranemodule (the inside of hollow fibers in the case of a hollow fibermembrane). While maintaining the intermembrane pressure difference at 50mmHg, the quantity of permeated water was measured. From this data, thewater permeability (l/m².hr.mmHg) was determined.

(3) Water permeability by the alcohol-dependent hydrophilizing method:

Ethanol was fed under pressure at a flow rate of 25 ml/min for 15minutes from one side of a test membrane module (the inside of hollowfibers in the case of a hollow fiber membrane) which had not beensubjected to any hydrophilizing treatment, whereby the porous membranewas wet fully to the interior of its pores with ethanol. Thereafter,water was caused to flow at a flow rate of 100 ml/min for 15 minutes sothat the ethanol contained within the pores was substituted by water.The water permeability was then measured by the method described in theabove testing method (2).

(4) Amount of polymer held:

Each polymer held on porous polyolefin membrane was measured in terms ofwt. % based on the unit weight of the membrane.

(5) Surface coverage (%):

Each porous membrane was immersed for 1 minute in the standard solution(blue) for wetting tests described in JIS K6768(1971), and having asurface tension of 54 dyn/cm. Thereafter, the membrane was dried in air,and a transverse cross-section of the porous membrane was observed at 10points through an optical microscope so as to determine the percentage(%) of the colored part relative to the membrane thickness. The valuewas employed as the surface coverage (%).

(6) Flex fatigue test:

While applying a load of 50 g/fil to a hollow fiber, the number ofbending strokes (bending angle: 90° ) required until the hollow fiberruptured was counted.

EXAMPLE 1

A membrane module having an effective membrane area of 163 cm² wasfabricated by using porous polyethylene hollow fibers, which hadslit-like pores having an average width of 0.5 μm and an average lengthof 2 μm, a porosity of 68%, a membrane thickness of 70 μm, an innerdiameter of 270 μm, and a water permeability of 1.3 l/m².hr.mmHg asmeasured by the alcohol-dependent hydrophilizing method. From the insideof the hollow fibers of the membrane module, a solution of thecomposition given in Table 1 was caused to flow under pressure at 7.5ml/min for 10 minutes. Thereafter, the membrane module was put in anitrogen gas atmosphere, in which extra solution was removed. Themembrane module was then dried for 16 hours in air. The membrane modulewas thereafter subjected to a heat treatment at 60° C. for 30 minutes ina nitrogen gas atmosphere, followed by thorough washing with acetone toobtain a hydrophilized porous membrane of this invention.

Subsequently measured were the amount of the resulting polymer held onthe porous membrane and the water penetration pressures after theimpartation of hydrophilicity.

In order to evaluate the durability of the hydrophilicity imparted bythe hydrophilizing treatment, water was caused to flow at 600 ml per cm²through the test membrane module after the hydrophilizing treatment.Thereafter, the test membrane module was dried and the water penetrationpressure was measured. These operation were repeated five times. Thewater penetration pressure after the 5 cycles of the water-washing anddrying was exactly the same as the water penetration pressure before thewater-washing and drying. The durability of the product of the presentExample was hence confirmed.

The results are summarized in Table 1.

EXAMPLES 2-6

Porous polyethylene hollow fibers, each of which had slit-like poreshaving an average width of 0.5 μm and an average length of 2 μm, aporosity of 68%, a membrane thickness of 60 μm, an inner diameter of 270μm, and a water permeability of 1.3 l/m² hr.mmHg as measured by thealcohol-dependent hydrophilizing method, were separately immersed for 3seconds in solutions of the compositions given in Table 1. The hollowfibers were thereafter taken out of their corresponding solutions andput in a nitrogen gas atmosphere, where extra solutions were removed.After drying for 16 hours in air, each of the porous hollow fibers wereheat-treated at 60° C. for 30 minutes in a nitrogen gas atmosphere sothat polymers were held on the pore walls of the corresponding poroushollow fibers. The porous hollow fibers were then thoroughly washed withacetone to obtain hydrophilized porous membranes of this invention. Theamounts of the polymers held on the hollow fibers and the waterpenetration pressures of the hollow fibers were measured.

Regarding the hydrophilized porous hollow fibers of Examples 2 and 4,their surface coverages (%) were measured and flex fatigue tests wereperformed thereon. Furthermore, their durability was also evaluated. Thesurface coverages (%) of the porous hollow fibers of Examples 2 and 4were 90-100% and 95-100%, respectively, thereby indicating that polymerlayers had been formed substantially on their entire pore walls. Thenumber of bending strokes until rupture were about 45,000 strokes and48,000 strokes, respectively. Compared with 50,000 strokes of anuntreated porous polyethylene hollow fiber, these values are notsignificantly different. It was hence confirmed that the strength of thehydrophilized porous hollow fibers was not changed substantially.Similar to Example 1, the water-washing and drying operation was alsoperformed 5 times to evaluate their durability. Both hollow fibers gavegood results.

EXAMPLES 7 & 8

Porous polyethylene hollow fibers of the same type as that employed inExample 2 were separately immersed for 5 seconds in their correspondingsolutions of the compositions shown in Table 1. They were then taken outof their corresponding solutions and put in a nitrogen gas atmosphere,in which extra solutions were removed. After being dried for 16 hours inair, each of the porous hollow fibers were exposed at a distance of 20cm for 3 seconds to a high-pressure mercury vapor lamp (rated powerinput: 80 W/cm) in a nitrogen gas atmosphere so that the surface-activemonomers were polymerized. The resulting porous hollow fibers were thenthoroughly washed with acetone to obtain hydrophilized porous membranesof this invention.

The amounts of the polymer held on their corresponding porous membranesand the water penetration pressures of the porous membranes weremeasured. Their durability was also evaluated in the same manner as inExample 2. Good results were obtained.

The results are also shown in Table 1.

However, the number of bending strokes until rupture in their flexfatigue tests were about 500 times and about 300 times, respectively.Compared with the porous membrane produced by the heat polymerizationtechnique in Example 2, the strength of the matrix of each of the porousmembranes was reduced significantly.

EXAMPLE 9

A hydrophilized porous membrane was obtained in the same manner as inExample 2 except that, instead of heat polymerization, polymerizationwas conducted by using an electron irradiation apparatus and byirradiating electron beams to the porous membrane at 20 Mrad at anacceleration voltage of 200 KV, an electron stream of 8.1 mA and atemperature of 80° C. or lower. The amount of the resultant polymer heldon the porous membrane and the water penetration pressure were measured.As shown in Table 1, relatively good results were obtained. On the otherhand, the number of bending strokes required until rupture in a flexfatigue test was about 100 strokes. The strength of the matrix of theporous membrane was also significantly reduced in this Example.

EXAMPLE 10

A planar porous polyethylene membrane, which had slit-like pores havingan average width of 0.8 μm and an average length of 3 μm, a porosity of70%, a membrane thickness of 42 μm, and a water permeability of 3.5l/m².hr.mmHg as measured by the alcohol-dependent hydrophilizing method,was immersed for 3 seconds in a solution of the composition given inTable 1. The membrane was thereafter taken out of the solution and putin a nitrogen gas atmosphere, where extra solution was removed. It wasthereafter dried for 16 hours in air.

The porous membrane was thereafter heat-treated at 70° C. for 20 minutesin a nitrogen gas atmosphere, followed by thorough washing with acetoneto obtain a hydrophilized porous membrane of this invention. The amountof the polymer held on the membrane and the water penetration pressurewere measured.

The results are shown in Table 1.

EXAMPLE 11

Porous polypropylene hollow fibers, which had slit-like pores having anaverage width of 0.2 μm and an average length of 0.8 μm, a porosity of45%, a membrane thickness of 22 μm, an inner diameter of 200 μm and awater permeability of 0.8 μ/m².hr.mmHg as measured by thealcohol-dependent hydrophilizing method, was immersed for 5 seconds in asolution of the composition given in Table 1. The hollow fibers werethereafter taken out of the solution and put in a nitrogen gasatmosphere, where extra solution was removed. They were thereafter driedfor 16 hours in air. The hollow fibers were then subjected to a heattreatment at 80° C. for 20 minutes in a nitrogen gas atmosphere,followed by thorough washing with acetone to obtain a hydrophilizedporous membrane of this invention.

Thereafter, it was evaluated in the same manner as in Example 2.

The results are shown in Table 1.

COMPARATIVE EXAMPLES 1 & 2

Polymer-holding porous membranes were obtained in exactly the samemanner as in Example 2 except for the use of a solution of thecomposition given in Table 1. Their performance was evaluated and theresults are shown in Table 1. The water penetration pressures were 8.3kg/cm² and 10.7 kg/cm², respectively, considerably higher than 0.2 kg/cmin Example 2. It is hence clear that these porous membranes did not haveadequate hydrophilicity. In addition, the surface coverages (%) of theporous membranes of Comparative Examples 1 and 2 were 4-8% and 0-2%,respectively.

COMPARATIVE EXAMPLES 3

A porous membrane with a polymer held thereon was obtained in exactlythe same manner as in Example 11 except that a solution of thecomposition given in Table 1 was used. Its performance was evaluated andthe results are shown in Table 1. Its water penetration pressure was11.3 kg/cm², which is a significantly high value compared with 0.5kg/cm² in Example 11. It is hence understood that the porous membranedid not have adequate hydrophilicity.

COMPARATIVE EXAMPLE 4

Porous polyethylene hollow fibers of the same type as that used inExample 2 was immersed for 1 minute in a solution of the compositiongiven in Table 1. The hollow fibers were then taken out of the solutionand was dried at room temperature for 16 hours. The hollow fibers werethen fabricated into a membrane module. The water penetration pressureof the membrane module was measured. It was found to be 0.2 kg/cm²,which indicated that hydrophilicity had been imparted by the adhesion ofthe surface-active monomer. When a water-washing and drying test waseffected once in the same manner as in Example 2, the water penetrationpressure increased to 8.0 kg/cm². It is hence clear that the adhesionmethod could impart hydrophilicity only temporarily.

                                      TABLE 1                                     __________________________________________________________________________                                               Water penetration                                                             pressure (kg/cm.sup.2)                                                        Before                                                                             After                                                                              After                                                                              Amount                                              Polymerization                                                                           imparta-                                                                           imparta-                                                                           water-                                                                             of                  Type of porous                                                                            Surface-active monomer                                                                     Solvent                                                                              catalyst   tion tion washing                                                                            polymer             Ex. polyolefin    HLB wt.    wt.        wt.                                                                              of hydro-                                                                          of hydro-                                                                          and  held                No. membrane                                                                              Kind  value                                                                             parts                                                                            Kind                                                                              parts                                                                            Kind    parts                                                                            philicity                                                                          philicity                                                                          drying                                                                             (wt.                __________________________________________________________________________                                                              %)                  Ex. 1                                                                             Polyethylene                                                                          Compound                                                                            9.2 100                                                                              Water                                                                             1000                                                                             Benzoyl 5  11.0 0.15  0.15                                                                              7.2                     hollow fiber                                                                          of formula          peroxide                                                  (1)                                                               Ex. 2                                                                             Polyethylene                                                                          Compound                                                                            9.2 100                                                                              Ace-                                                                              1000                                                                             Benzoyl 10 11.0 0.2  0.2  8.1                     hollow fiber                                                                          of formula   tone   peroxide                                                  (1)                                                               Ex. 3                                                                             Polyethylene                                                                          Compound                                                                            17.2                                                                              100                                                                              Ace-                                                                              1000                                                                             Benzoyl 10 11.0 5.0  --   2.8                     hollow fiber                                                                          of formula   tone   peroxide                                                  (2)                                                               Ex. 4                                                                             Polyethylene                                                                          Compound                                                                            6.5 100                                                                              Ace-                                                                              1000                                                                             Benzoyl 10 11.0 0.7  0.7  14.9                    hollow fiber                                                                          of formula   tone   peroxide                                                  (3)                                                               Ex. 5                                                                             Polyethylene                                                                          Compound                                                                            5.8 100                                                                              Ace-                                                                              1000                                                                             Benzoyl 5  11.0 0.7  --   12.1                    hollow fiber                                                                          of formula   tone   peroxide                                                  (4)                                                               Ex. 6                                                                             Polyethylene                                                                          Compound                                                                            3.1 100                                                                              Ace-                                                                              1000                                                                             Benzoyl 10 11.0 3.9  --   13.0                    hollow fiber                                                                          of formula   tone   peroxide                                                  (5)                                                               Ex. 7                                                                             Polyethylene                                                                          Compound                                                                            9.2 100                                                                              Ace-                                                                              1000                                                                             Chlorobenzo-                                                                          3  11.0 0.6  0.7  4.1                     hollow fiber                                                                          of formula   tone   phenone                                                   (1)                                                               Ex. 8                                                                             Polyethylene                                                                          Compound                                                                            6.5 100                                                                              Ace-                                                                              1000                                                                             Chlorobenzo-                                                                          3  11.0 1.3  1.4  6.5                     hollow fiber                                                                          of formula   tone   phenone                                                   (3)                                                               Ex. 9                                                                             Polyethylene                                                                          Compound                                                                            9.2 100                                                                              Ace-                                                                              1000                                                                             Chlorobenzo-                                                                          3  11.0 0.4  --   4.7                     hollow fiber                                                                          of formula   tone   phenone                                                   (1)                                                               Ex. 10                                                                            Planar poly-                                                                          Compound                                                                            10.3                                                                              100                                                                              Ace-                                                                              2000                                                                             Chlorobenzo-                                                                          5   4.5 0.2  --   7.7                     ethylene                                                                              of formula   tone   phenone                                           membrane                                                                               (6)                                                              Ex. 11                                                                            Polypropylene                                                                         Compound                                                                            12.6                                                                              100                                                                              Methyl                                                                            1500                                                                             Azobisiso-                                                                            3  15.0 0.5  0.5  11.8                    hollow fiber                                                                          of formula   ethyl  butyronitrile                                             (7)          ketone                                               Comp.                                                                             Polyethylene                                                                          Acrylic                                                                             37  100                                                                              Ace-                                                                              1000                                                                             Benzoyl 10 11.0 8.3  --   --                  Ex. 1                                                                             hollow fiber                                                                          acid         tone   peroxide                                      Comp.                                                                             Polyethylene                                                                          Compound                                                                            0.5 100                                                                              Ace-                                                                              1000                                                                             Benzoyl 10 11.0 10.7 --   --                  Ex. 2                                                                             hollow fiber                                                                          of formula   tone   peroxide                                                  (8)                                                               Comp.                                                                             Polypropylene                                                                         4-Vinyl-                                                                            28  100                                                                              Methyl                                                                            1500                                                                             Azobisiso-                                                                            3  15.0 11.3 --   --                  Ex. 3                                                                             hollow fiber                                                                          pyridine     ethyl  butyronitrile                                                          ketone                                               Comp.                                                                             Polyethylene                                                                          Compound                                                                            9.2 100                                                                              Ace-                                                                              1000                                                                             --      -- 11.0 0.2  8.0  --                  Ex. 4                                                                             hollow fiber                                                                          of formula   tone                                                             (1)                                                               __________________________________________________________________________     Note:                                                                         (1) H.sub.2 CCHCOO(EO).sub.12 (PO).sub.20 (EO).sub.12 CH.sub.3                (2) H.sub.2 CCHCOO(EO).sub.14 COCHCH.sub.2                                    (3) H.sub.2 CCHCOO((CH.sub.2).sub.4 O).sub.6 COCHCH.sub.2                     (4) H.sub.2 CCHCOO(PO).sub. 50 (EO).sub.13 COCHCH.sub.2                       (5) H.sub.2 CCHCOO(PO).sub.50 (EO).sub.5 COCHCH.sub.2                         (6) H.sub.2 CCHCOO(EO).sub.12 (PO).sub.20 (EO).sub.12 COCHCH.sub.2            ##STR4##                                                                      (8) H.sub.2 CCHCOO(PO).sub.55 COCHCH.sub.2                                    ##STR5##                                                                      ##STR6##                                                                 

EXAMPLE 12

A planar poly-4-methyl-1-pentene porous membrane which had slit-likepores having an average width of 0.2 μm and an average length of 0.5 μm,a porosity of 43%, a membrane thickness of 35 μm, and a waterpermeability of 0.2 l/m².hr.mmHg as measured by the alcohol-dependenthydrophilizing method, was immersed for 3 seconds in a solution of thecomposition given in Table 2. The membrane was thereafter taken out ofthe solution and put in a nitrogen gas atmosphere. It was thereafterdried for 10 minutes to remove extra solution. The membrane was thenheated and polymerized at 75° C. for 25 minutes in a nitrogen gasatmosphere, followed by its thorough washing with acetone to obtain ahydrophilized porous membrane of this invention.

Thereafter, its performance was evaluated in the same manner as inExample 2. The results are shown in Table 2.

EXAMPLE 13

A polymer-holding porous membrane was obtained in exactly the samemanner as in Example 2 except for the use of a solution of thesurface-active monomer given in Table 2. Its performance was evaluatedand the results are shown in Table 2.

EXAMPLE 14

A porous polyethylene hollow fiber of the same type as that employed inExample 1 was fed continuously at a speed of 2 m/min into a solutionbath of 10 cm long, whereby the hollow fiber was subjected to a dippingtreatment. In a first pipe 2 cm in diameter and 4 m long, theaccompanying extra solution was removed and the hollow fiber was dried.Heat polymerization was then effected in a second pipe 2 cm in diameterand 3 m long.

The solution shown in Table 2 was filled in the solution bath. Nitrogengas of room temperature and hot nitrogen gas of 80° C. were caused toflow at 20 l/min through the first and second pipes respectively

Thereafter, the hollow fiber was thoroughly washed in an aqueoussolution which contained 50 vol.% of ethanol and was then washed inwater of 60° C. to obtain a hydrophilized porous membrane of thisinvention.

The surface coverage (%) of the porous membrane was 95-100%. It wasobserved that the resultant polymer had been formed substantially overthe entire pore walls. Further, the number of bending strokes untilrupture was about 4,900 times. This number did not change substantiallycompared with that before the treatment. In the same manner as inExample 1, 5 cycles of water-washing and drying treatments were effectedto evaluate the durability. Good results were obtained.

These results are shown in Table 2.

EXAMPLE 15

A hydrophilized porous membrane according to this invention was obtainedin exactly the same manner as in Example 14 except that the compositionof the solution was changed to that shown in Table 2.

The surface coverage (%) of the porous membrane was 95-100%. It wasobserved that the resultant polymer had been formed substantially overthe entire pore walls. Further, the number of bending strokes untilrupture was about 5,000 strokes. This number did not changesubstantially compared with that before the treatment. In the samemanner as in Example 1, 5 cycles of water-washing and drying treatmentswere effected to evaluate the durability. Good results were obtained.

The results are shown in Table 2.

EXAMPLE 16

A hydrophilized porous membrane according to this invention was obtainedby using a porous polypropylene hollow fibers of the same type as thatemployed in Example 11 and the solution given in Table 2 in the samemanner as in Example 14 except that the temperature of the hot nitrogengas was maintained at 85° C. and the flow rate was set at 10 l/min.

EXAMPLE 17

A hydrophilized porous membrane according to this invention was obtainedby using porous poly-4-methyl-1-pentene hollow fibers, which hadslit-like pores having an average width of 0.2 μm and an average lengthof 0.5 μm, a porosity of 40%, a membrane thickness of 35 μm, an internaldiameter of 200 μm, and a water permeability of 0.2 μ/m².hr.mmHg asmeasured by the alcohol-dependent hydrophilizing method, and thesolution given in Table 2 in the same manner as in Example 14 exceptthat the temperature of the hot nitrogen gas was maintained at 90° C.,the flow rate of the hot nitrogen was set at 30 l/min and the feedingspeed of the hollow fiber was controlled at 3 m/min. The performance ofthe membrane was evaluated. The results are shown in Table 2.

As apparent from the above Examples, it is clear that the porousmembranes according to this invention had extremely low waterpenetration pressures and superior hydrophilicity compared with theuntreated porous polyolefin membranes. In the evaluation of theirdurability by the repetition of the water-washing and drying treatments,their water penetration pressures did not change substantially. It ishence clear that the porous membranes of this invention hadhydrophilicity having excellent durability.

                                      TABLE 2                                     __________________________________________________________________________                                               Water penetration pressure                                                    (kg/cm.sup.2)                                                                 Before                                                                             After                                                                              After                                                                              Amount              Type of                                    impar-                                                                             impar-                                                                             water-                                                                             of                  porous    Surface-active monomer                                                                      Solvent                                                                             Polymerization catalyst                                                                    tation                                                                             tation                                                                             washing                                                                            polymer             Ex.                                                                              polyolefin    HLB wt.   wt.          wt.                                                                              of hydro-                                                                          of hydro-                                                                          and  held                No.                                                                              membrane                                                                             Kind   value                                                                             parts                                                                            Kind                                                                             parts                                                                            Kind      parts                                                                            philicity                                                                          philicity                                                                          drying                                                                             (wt.                __________________________________________________________________________                                                              %)                  Ex.                                                                              Poly-4-                                                                              Compound of                                                                          9.2 100                                                                              Ace-                                                                             1000                                                                             Benzoyl peroxide                                                                        10 12.8 0.5  --   4.7                 12 methyl-                                                                              formula (6)   tone                                                     1-pentene                                                                     planar                                                                        membrane                                                                   Ex.                                                                              Polyethylene                                                                         Compound of                                                                          5.2 100                                                                              Ace-                                                                             1000                                                                             "         10 11.0 0.4  --   7.5                 13 hollow fiber                                                                         formula (9)   tone                                                  Ex.                                                                              Polyethylene                                                                         Compound of                                                                          9.2 100                                                                              Ace-                                                                             1000                                                                             Bis(4-t-butyl-                                                                          10 11.0 0.3  0.3  7.0                 14 hollow fiber                                                                         formula (1)   tone  cyclohexyl)                                                                   peroxy dicarbonate                              Ex.                                                                              Polyethylene                                                                         Compound of                                                                          5.8 100                                                                              Ace-                                                                             1000                                                                             Bis(4-t-butyl-                                                                          10 11.0 0.6  0.7  13.3                15 hollow fiber                                                                         formula (5)   tone  cyclohexyl)                                                                   peroxy dicarbonate                              Ex.                                                                              Poly-  Compound of                                                                          9.2 100                                                                              Ace-                                                                             1000                                                                             Bis(4-t-butyl-                                                                          10 15.0 0.4  0.4  12.0                16 propylene                                                                            formula (1)   tone  cyclohexyl)                                        hollow fiber               peroxy dicarbonate                              Ex.                                                                              Poly-4-                                                                              Compound of                                                                          9.2 100                                                                              Ace-                                                                             1000                                                                             Benzoyl peroxide                                                                        10 12.8 0.5  0.5  4.8                 17 methyl-                                                                              formula (1)   tone                                                     1-pentene                                                                     hollow fiber                                                               __________________________________________________________________________

What is claimed is:
 1. A hydrophilized porous polyolefin membrane,wherein a polymer of a monomer containing at least one unsaturatedpolymerizable bond and having an HLB value of 2-20 is held on at least apart of the pore walls of a starting porous polyolefin membrane.
 2. Thehydrophilized porous polyolefin membrane as claimed in claim 1, whereinthe HLB value of the monomer is 5-15.
 3. The hydrophilized porouspolyolefin membrane as claimed in claim 1, wherein the starting porousmembrane is in the form of hollow fibers.
 4. The hydrophilized porouspolyolefin membrane as claimed in claim 1, wherein the polyolefin is apolymer composed principally of at least one monomer selected from thegroup consisting of ethylene, propylene, 4-methyl-1-pentene and3-methyl-1-butene.